Add branch date guidance to documentation.

This is a set of guidelines that the Blink/Web Platform team leads
came up with, based on experience and input from TPMs.

R=amineer@chromium.org
BUG=

Change-Id: Iff53ce441ab46bd52ada5285ad94af57dce27b18
Reviewed-on: https://chromium-review.googlesource.com/693345
Reviewed-by: Alex Mineer <amineer@chromium.org>
Commit-Queue: Dimitri Glazkov <dglazkov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511203}

docs/README.md

@@ -135,6 +135,9 @@ used when committed.
     features in Chromium
 *   [Adding a new feature flag in chrome://flags](how_to_add_your_feature_flag.md) - Quick
     guide to add a new feature flag to experiment your feature.
+*   [Guidelines for considering branch dates in project planning](release_branch_guidance.md) -
+    What to do (and not to do) around branch dates when scheduling your project
+    work.
 
 ### Testing
 *   [Running and Debugging Layout Tests](testing/layout_tests.md)

docs/process/release_cycle.md

@@ -45,7 +45,8 @@ is cut as the release build. All ReleaseBlock-Stable bugs should be fixed by the
 stable cut, which consequently corresponds to the absolute last date a merge to
 the release branch should be taken for inclusion in the initial stable release.
 The stable cut will, unless there are exceptions, use the final beta as its
-basis. For Chrome OS, the stable cut date may be different than the rest of Chrome.
+basis. For Chrome OS, the stable cut date may be different than the rest of
+Chrome.
 
 ### Stable Release
 
@@ -75,3 +76,7 @@ Except for extremely critical issues (e.g. security or privacy escalations),
 a Chrome stable refresh should not be pushed more than **2 weeks** after the
 initial build has been released to the public. In this case, we should consider
 punting any detected production issues to the next release.
+
+## More Info
+
+For more information, see these [guidelines for considering branch dates in project planning](../release_branch_guidance.md).

docs/release_branch_guidance.md

@@ -0,0 +1,22 @@
+# Guidelines for Considering Branch Date in Project Planning
+
+*   Avoid changes to persisted data (e.g. data serialization, database schemas,
+    etc) just before branch due to the risk of reverts. If you modify data that
+    will be synced, this change should be forward compatible for up to four
+    milestones (as stable and canary are at times four milestones apart and it's
+    easonable to assume that somebody runs stable on chromeos and canary on mac,
+    both syncing to the same account).
+*   Two weeks prior to the branch point, avoid committing big and risky changes
+    or enabling non-trivial features.
+*   Pay more attention to complexity and structure of CLs around branch points,
+    in case fixes need to be merged back after branch is created. Break up a CL
+    into smaller pieces to facilitate potential merges.
+*   As the branch point gets closer, consider if there is data to gather (like
+    adding use counters, or other histograms that we want) and prioritize that
+    work. Metrics changes on branch don't meet the bar.
+*   Prioritize stability and "bake time", as well as other ways to reduce the
+    release risk in your timeline discussions.
+*   Consider API adoption timelines: code may not be exercised until developers
+    adopt API, so feature freeze and branch could have minimal impact on
+    stability.
+